Very Shallow Loamy Foothills
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition 1A More details
- Transition 1B More details
- Transition 2A More details
- Transition 3A More details
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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Description
State 1 represents the reference community and describes the ecological potential for this site.
States and Community Phases included in this document include those previously recognized by Fire Resource Assessment Program (State of California, FRAP, 1998) and other entities, as a result of the use of ordination software and professional consensus (Allen-Diaz et al., 1989; Vayssieres and Plant, 1998 and George et al., 1993).
The reference state for this ecological site has one community phase. Found on dry south and southwest-facing slopes, this site has a very sparse blue oak overstory is primarily composed of a grass and forb understory with scattered trees and shrubs. Representation of the Reference State is lacking except on the steepest slopes due the impacts of past management.
Long fire-free periods have resulted in a shrub component that is often mature and even decadent, with low natural regeneration. While it has been demonstrated that buckbrush can regenerate in the absence of fire (League, 2005), long-term fire suppression or exclusion has led to eventual decadence and a decline in shrub abundance.
Oaks are efficient water users, adapted to very low moisture conditions by virtue of their small leaf size, the regulation of water loss through the leaf stomata and by tapping into water below fractured rock (Baldocchi et al, 2007). Trees and shrubs on this site compete intensively for limited available water and growing space.
Shrub cover slows water runoff rates, improving water storage. This available water may be present later in the growing season due to decreased evaporation and shading, maintaining moisture longer than under just grasses alone (Gill and Burke, 1999). Some deeply rooted trees and shrubs may also induce hydraulic lift, transporting water to the upper soil layers (Richards and Cadewell, 1987, Caldwell et al, 1998, Ishikawa and Bledsoe, 2000, Liste and White, 2008), supporting the development of neighboring plants. Nutrients are also concentrated around shrub bases from litter fall and from sediment capture via movement of soil particles.
The duration of vegetation successional stages varies greatly, and lacks sufficient research to gain better estimates. It has been estimated by some research that for some similar ecological sites, it may take at least 50 years (Mayer and Laudenslayer, 1988). Blue oak growth is slow and variable. Most stands of blue oak range from 80 to 100 years of age (Kertis et al, 1993). There are remnant older blue oak specimens that may range to over 450 years of age (Stahle et al, 2013) in more remote or steep locations. Mature brush development can take 10 to 15 years.
Wildlife may have limited use of this site for foraging during the spring; resting or escape cover are generally lacking.
Submodel
Description
This is the state most widely represented on the landscape. The Annual-Dominated Grassland State is greatly influenced by the timing and amount of precipitation and the amount of residual dry matter (George et al., 2001a). Currently species composition and productivity of the annual-dominated grassland and understory grasses and forbs vary greatly within and between years.
Annual dominated systems have shallow root structures that dry out quickly during rapid spring growth and evapotranspiration, quickly depleting soil moisture. Annuals use available water primarily in the top 1 foot of soil, however filaree and other forbs may also be able to use a substantial amount of water at soil depths (George et al., 2001). Although nutrient leaching from grassland systems is variable, nutrients that are moved beyond the shallow root systems of the annual grasses are lost to leaching. Water infiltration may be more rapid in grasslands than in shrub-dominated landscapes. Low soil cover contributes to increased erosion on this ecological site.
Submodel
Description
Loss of blue oak through conversion, severe fire or poor oak regeneration and senescence of mature oaks could lead to a shrub-dominated state. Shrub species such as buckbrush require moderate to severe fire to scarify the seed coat to produce abundant regeneration.
Available water may be present later in the growing season under shrubs due to decreased evaporation and shading, maintaining moisture longer than under just grasses and forbs alone (Gill and Burke, 1999). Some deeply rooted trees and shrubs may also induce hydraulic lift, transporting water to the upper soil layers (Richards and Cadewell, 1987, Caldwell et al, 1998, Ishikawa and Bledsoe, 2000, Liste and White, 2008), supporting the development of neighboring plants. Nutrients are concentrated around shrub bases from litter fall and from sediment capture via movement of soil particles.
As development of the shrub community progresses, inter-shrub native and non-native herbaceous vegetation decreases, and less understory vegetation is remaining.
Submodel
Mechanism
Continuous grazing and clearing are triggers that may eliminate blue oak and buckbrush across the landscape. Indicators of a change in structure and function on this site would include a lack of reproduction in shrubs and trees and a reduction in cover. Insect predators and high temperatures may result in significant buckbrush seed mortality and reduction of the seedbank (O’Neil and Parker, 2005). Seed of buckbrush is also highly preferred by small mammals (League, 2004), and the low level of non-fire recruitment of buckbrush on this site may also be a function of very low soil moisture.
Mechanism
Moderate to severe fire or conversion are the triggers that kill blue oak and stimulates buckbrush seed to germinate by scarifying the seed “banked” in the soil (League, 2005). While buckbrush may have very limited natural regeneration without fire, fire-stimulated regeneration is more abundant; historic photos of the area indicate that shrub density was previously of greater extent with more frequent fire than under current conditions.
Mechanism
Moderate to severe fire stimulates buckbrush seed to germinate. Buckbrush usually requires fire in order to scarify the seed “banked” in the soil (League, 2005). Mechanical disturbance also may also occasionally act to scarify seed and facilitate germination, though to a much lesser degree (Bonner et al., 2008).
First year survival of buckbrush seedlings may be very low (League, 2005) due to drought conditions.
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